Bug 24244: Send null instead of empty string if branch not set

Bug 24244: Send null instead of empty string if branch not set

Fix NewSuggestion if branchcode is passed with an empty string.
Error was "Broken FK constraint"

Test plan:
Create a new suggestion, select "any" for the library.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Joy Nelson <joy@bywatersolutions.com>